Electrical pulse operated laser sampling light amplifier

ABSTRACT

A sub-nanosecond pulse operated semiconductor laser amplifier which admits light input for a time interval after the commencement of the electrical pulse. The duration of the said time interval is shorter than that of the said electrical pulse. Amplified light output reaches a peak value after the termination of the said electrical pulse. The pulsed operation can be repeated a few billion times per second. Applications in a fiber-optics communication system, a bar code scanner, and in a super speed camera are described.

BACKGROUND OF THE INVENTION

1 Field of the Invention

The present invention relates to semiconductor laser amplifiers whichcan be used as a repeater or an amplifying distributor on-line switch ina time-division multiplexing optical fiber communication system. Otherapplications include bar code scanners and a super speed video camerawhich can take billions of pictures per second with a shutter speed ofapproximately 10⁻¹¹ second.

2. Description of Prior Art

Presently there are two types of semiconductor laser light amplifiers:(i) the traveling wave amplifier (TWA) and (ii) the resonant type orFabry Perot amplifier (FPA). In both types, direct currents are used toinject minority carriers into the active channel for lasing action. Theessential difference between the two types is the reflectivities at theends of the active channel.

In a TWA, the reflectivities are made as small as possible. Considerableamount of research work is still being performed with respect toreducing reflectivities. The input light wave is amplified as it travelsfrom one end of the active channel to the other end. The reflected wavesare also amplified. If the product of the reflectivities and the roundtrip gain exceeds 1, the "amplifier" would emit light without any lightinput creating a useless amplifier. If the product is a significantfraction of 1, an interference pattern would develop between the mainwave and the reflected waves. The interference pattern is "noise" and isa severe limiting factor on the gain of a TWA.

In an FPA, the reflectivities are usually on the order of a few tenths.Thus, light photons are partially trapped between the two reflectingends, and their number multiplies due to stimulated emission as thephotons travel back and forth in the active channel. The amplificationfactor increases exponentially with the densities of the minoritycarriers in the active channel. At a certain density level, the productof the reflectivities and the round trip amplification factor is equalto 1, and the "certain density level" is then referred to in thetechnical literature as the critical density which is denoted here asN_(c). It should be noted that the operating minority carrier densitylevel in a TWA can be considerably higher than the critical density ofan FPA made with the same material because the reflectivities of the TWAare much lower.

Operating at its critical density, the total loss of photons in an FPA,including photon losses at its two ends, is substantially equal to thenumber of newly generated photons due to stimulated emission. If thedevice is left alone without further light input, the number of photonstravelling back and forth in its active channel would remain constant,with a constant output at the output facet. The device would not beuseful as a light amplifier because it would provide an output withoutany input.

However, if an FPA is operated at a carrier density level N slightlybelow critical, (N<N_(c), N_(c) -N<<1), and a light signal is applied toits input, the photon density in the active channel would build upgradually, until the net photon loss is equal to the number of inputphotons. The following would happen as N approaches N_(c) :

(i) the amplifier gain increases;

(ii) the buildup time, or response time increases; and

(iii) the amplifier bandwidth narrows.

There is a limit as to how close N can be made to approach N_(c).Because each stimulated emission means the loss of one minority carrier,with the same injection dc current, N reduces as the light inputincreases. The effect of gain reduction with input light intensity iscalled "gain compression" and is common to all laser light amplifiers.However, the gain reduction is far more severe in an FPA because itsgain depends on 1/(N_(c) -N) rather than on N itself.

The present invention provides a higher performance than both the TWAand the FPA because it does not require the following limitingconditions of operation:

(i) vanishingly small reflectivities at the two end facets; and

(ii) an operating minority carrier density less than the criticaldensity but approaching the critical density.

In addition, the signal sampling effect which is characteristic of thepresent invention is important to the applications mentioned inparagraph 1, Field of the Invention.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a pulse operatedsemiconductor laser light amplifier which does not require extremelysmall reflectivities at the two end facets.

It is another object of the present invention to provide a pulseoperated semiconductor laser light amplifier which is not restricted byan operating minority carrier density less than the critical density.

It is an object of the present invention to provide a pulsed FPA havinga higher signal to noise ratio and higher gain as compared to thepresent semiconductor laser amplifiers.

It is yet a further object of the present invention to provide a pulseoperated semiconductor laser amplifier which admits light input for atime interval after the commencement of the electric pulse therebyproducing a signal sampling effect on the input light signal.

In accordance with the present invention, a pulsed laser sampling lightamplifier includes a buried heterostructure semiconductor laser with aresonant cavity which is adapted to be connected to a source of electriccurrent pulses. The current pulses are applied to the semiconductorlaser which creates minority carriers within the resonant cavity.

The current source should create pulses of sufficient magnitude andduration to allow the minority carrier to momentarily rise above itscritical density after the pulse is terminated. Also, the current pulsesshould be sufficiently separated to allow the minority carriers to decaysignificantly below its critical density before the next current pulseis applied.

As a result of the present invention, a pulse operated semiconductorlaser has a signal sampling speed in the order of 0.02 nanoseconds. Thesignal sampling effect makes the amplifier less sensitive to reflectedsignals outside the sampling period as well as reducing signal noise byoperating only during the sampling period.

The pulse operated semiconductor laser has reflectivities of both endfacets of the active channel which exceed 0.1. The present inventionprovides both higher gain and signal to noise ratio.

A preferred form of the pulse operated laser sampling light amplifier,as well as other embodiments, objects, features and advantages of thisinvention, will be apparent from the following detailed description ofillustrative embodiments thereof, which is to be read in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electrical pulse operated lasersampling light amplifier consisting of a buried heterostructure laserand connected to an electric circuit to supply a current pulse orwaveform.

FIG. 2 is a vertical cross-section view of the buried heterostructurelaser amplifier taken along line 2--2.

FIG. 3 is a plot illustrating the relationship between a single currentpulse and the minority carrier density and photon density as a functionof time.

FIG. 4 is a plot illustrating the relationship between a dual peakcurrent pulse and the minority carrier density and photon density as afunction of time.

FIG. 5 is a vertical cross-section view of the buried heterostructurelaser amplifier modified to receive light input from air or empty spaceand including a light detecting diode.

FIG. 6 is an illustrative block diagram of a communication system formedin accordance with the present invention and having a single source ofinput light pulses being supplied to a plurality of controlled pulseoperated laser sampling light amplifiers.

FIG. 7(a)-(c) are plots illustrating the variety of operations which canbe performed by a plurality of controlled pulse operated laser samplinglight amplifiers in an optical communication system, in accordance withthe present invention.

FIG. 8 is an illustrative block diagram of a bar code scanner formed inaccordance with the present invention.

FIG. 9 is a block diagram of a super speed video camera (SSVC) formed inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a preferred embodiment of the invention. On the righthand side is an illustration of a buried heterostructure (BH) laseramplifier. The laser amplifier includes a thin GaAs slab 1 with typicaldimensions Δx =3 μm, Δy=0,1 μm, and Δz=100 μm. The slab 1 is embedded inGa_(1-x) Al_(x) As on all four sides. Since Ga_(1-x) Al_(x) As has alower index of refraction and larger energy gap, the slab 1 becomes anoptical waveguide as well as an active lasing channel (also referred toherein as a resonant cavity) provided sufficientminority carriers areinjected into it (N>N_(c)). Referring to FIG. 1, reference numeral 2denotes a negative terminal made of n-Gal_(1-x) Al_(x) As material.Reference numeral 3 denotes a positive terminal madeof p-Ga_(1-x) Al_(X)As material. Numerals 4 and 5 refer to the negative and positivecontacts respectively. Reference numeral 6 represents an oxideinsulation layer. Numeral 7 denotes a n-Ga_(1-x) Al_(x) As buryinglayer, and numeral 8 refers to a n-GaAs substrate. Theslab 1 issandwiched between the positive terminal 3 on the top of the slab, thenegative terminal 2 on the bottom of the slab, and on both sidesby theburying layer 7. Below the burying layer is a layer of n-GaAs substrate8 and below that is the negative contact 4. On top of the burying layer7 is an oxide insulation layer 6 and on top of that is the positivecontact 5. On the left side of FIG. 1, reference numeral 9 represents anelectrical circuit which supplies a current pulse of specified waveformand duration to the laser amplifier. The circuit includes a positiveterminal 10 and a negative terminal 11. In the preferred arrangement,pulse source 9 can be a separate unit, or part of an optico-electronicintegrated circuit included within the laser assemblyon the right sideof FIG. 1.

FIG. 2 is a vertical cross-section cut through the center of laseramplifier 1. The laser amplifier is shown in FIG. 2 as including endfacets 12, 13 situated at the opposite axial ends of slab 1. Light inputis through the end facet 12 of the active channel 1, and light output isthrough the end facet 13. Arrow A in FIG. 2 indicates the direction oflight output. In the preferred embodiment, both the reflectivities at 12and 13 are at least 0.1 or above. Thus, the present invention can bedescribed as a pulse operated FPA, or, PFPA.

FIG. 3 illustrates the changes in minority carrier density (representedby curve 15) and photon density (represented by curve 16) after acurrent pulse (represented by curve 14) is applied. Initially theminority carrierdensity is very much below its critical level: N-N_(c)<0. Any input light wave is attenuated. With the application of acurrent pulse, N risesfrom a level below N_(c) to a level above N_(c)and rises to a peak approximately at the time the current pulseterminates. After the current pulse terminates, N decays essentiallyexponentially towards zero and is accelerated in its decay by stimulatedemission. The number of coherent orsignal photons in the active channel1 rises exponentially at a rate proportional to N-N_(c) and reaches apeak at T₂ when N=N_(c). Shortly after T₁, the coherent photon densityin the active channel 1is so large compared to the photon input that thelatter becomes insignificant. Thus, there is a short interval in whichthe pulsed amplifier is most sensitive to input. This short interval isdenoted as T_(SO) in FIG. 3. Since the output light intensity isproportional to the photon density in the active channel 1, the gain ofthe amplifier can be expressed as ##EQU1##where K_(a) is a constantrepresenting the trapping effect in a pulse operated FPA, is the opticalmode confinement factor, A is the gain constant for accumulatedemissions, ν_(g) is the velocity of light inside the active channel andN is the minority carrier density.

In a traveling wave amplifier (TWA), an input light signal passesstraight through the active region. In contrast, the input light signalin the pulse operated FPA of the present invention is trapped betweenthe two reflecting surfaces. The signal accumulates as it is beingamplified. The constant K_(a) represents the signal trapping andaccumulating effect. The constant K_(a) significantly multiplies theoverall gain of the amplifier.

The PFPA of the present invention operates at an minority carrierdensity substantially above critical. It does not require a closely keptsub-critical minority carrier density, and consequently it does not havethe same disadvantages of an FPA. The reflectivities at both end facetsofthe PFPA of the present invention are integral parts of its operation.The PFPA does not need nearly zero reflectivities, and consequently, itsgain is not limited by such considerations. However, noise due tospontaneous emission may impose a limitation on gain.

In accordance with the present invention, a method for increasing thesignal to noise ratio in the amplifier is illustrated in FIG. 4. Thefirstpeak of the pulse increases the minority carrier density up to itscriticalvalue N_(c). The magnitude of the current pulse between the twopeaks is preferably such that the number of minority carriers ismaintained at the critical level N_(c) during the period between thepeaks. The second peak increases the minority carrier density above thecritical level N_(c). The electrical current pulse waveform 14 isgenerated with two peaks separated in time. Consequently, there is aperiod ΔT (generally between the peaks) in which N=N_(c) as shown inFIG. 4. Both the input signal photons and noise photons due tospontaneous emission areaccumulated during the period ΔT. Since thesignal photons are accumulated coherently and the noise photons areaccumulated incoherently,there is a substantial gain in the signal overnoise ratio.

When the pulsed laser light amplifier of the present invention is toreceive light input directly from air or empty space, the amplifier mayinclude a matching funnel section 17 of approximately the same index ofrefraction as active channel 1 which greatly increases the amplificationfactor and signal to noise ratio.

The laser amplifier of the present invention may further include a lightdetection diode 18 having adjacent n-type and p-type semiconductorsections 46 and 48, respectively, as shown in FIG. 5. The diode 18 ismounted adjacent to the output facet 13 such that the junction 50 of then-type and p-type sections 46, 48 is axially aligned with the outputfacet13 and the resonant cavity 1. An oxide insulation layer 58 isinterposed between the diode 18 and the positive and negative terminals3, 2 of the laser amplifier to insulate the diode from the amplifier.The diode 18 further includes positive and negative contacts 52, 54respectively coupled to the p-type and n-type sections 48, 46.Electrical wires 56 are connected to contacts 52, 54. The input lightfrom the active channel enters the junction 50 of the n-type and p-typesemiconductor sections 46,48 of the light detecting diode 18. The lightdetecting diode converts the light energy into an electrical outputwhich may be digitally processed. By pulsing the structure shown in FIG.5 with electrical current pulses, as described in relation to theembodiment of the pulsed light amplifier shown in FIG. 1, an enhancedlight detector may be realized.

FIG. 6 is a block diagram of a communication system formed in accordancewith the present invention utilizing a plurality of pulse operated laserlight amplifiers 19 operated by a controller 20, and FIG. 7 illustratesthat input light pulses may be distributed in a variety of methods bysucha system. Through the use of control signals from the controller 20to eachlaser amplifier 19, the system of laser amplifiers can be used asa switch or as a distributor, as will be explained in greater detail.

FIG. 8 is a block diagram of a bar code scanner and FIG. 9 is a blockdiagram of a super speed video camera, both of which will be explainedin greater detail in the following examples.

The following examples serve to provide further appreciation of theinvention but are not meant in any way to restrict the effective scopeof the invention.

EXAMPLE 1 Application of Pulse Operated Sampling Laser Light Amplifierto a Bar Code Scanner

Referring to FIG. 8, the improved bar code scanner replaces thepresently used light detector with an enhanced light detector 31, suchas the one shown in FIG. 5. The scanner electronic circuitry 21energizes the laser light source 24 with a pulse current signal (shownas being provided to light source 24 by line 22). The laser light source24 produces a light beam 25, which is focused by an electro-opticaldevice 26. The electro-optical device 26 is controlled by a signal 23from the scanner electronic circuitry 21. A scanning beam 27 isgenerated by electro-optical device 26 and is scattered, as shown bylines 29, by the bar coded surface 28. A portion 30 of the light beamreflected by the bar coded surface enters the input of the enhancedlight detector 31. The input light is amplified and converted to anelectric current signal whichis provided (as illustrated by line 32) tothe scanner electronic circuit 21 for processing. The scanner electroniccircuitry 21 also provides electric current signal pulses to theenhanced light detector 31, as described previously in relation to theembodiment shown in FIG. 5. The electric current pulses provided to theenhanced light detector 31 and thepulses of the current signal providedby the scanner circuit to the laser light source 24 are preferablysynchronized.

Except for prefiltering, the electrical pulses from the photo cell ordiode18 are processed digitally. The digital processing includes:

a) determining bar and space widths directly from pulse counts;

b) coordinating adjacent lines for a better determination of bar andspace widths;

c) algorithms for various corrections such as wide angle or warpedsymbols;and

d) symbol identification.

The advantages of such a scanner include low cost, high performance andlong life for the laser light source. The scanner is low cost sincethere is little analog electronics. All processing and control isperformed digitally with software.

The performance of the scanner is enhanced due to the large gain of thepulsed laser sampling amplifier of the present invention used in thescanner, which makes it possible to work with a very weak reflectedbeam. Because the laser light source is driven by a pulsed signal andthus may be operated at a very low duty cycle, its life is extended.

EXAMPLE 2 Super Speed Video Camera (SSVC)

The video camera employing the laser amplifier of the present inventionwould have a shutter speed of approximately 0.02 nanoseconds (2×10⁻¹¹sec. ). The camera can take as many as five billion (5×10⁹) pictures persecond.

Referring to FIG. 9, the SSVC contains a lens system 34 which focuses apicture (shown by line 35) prior to the picture 35 entering an array ofenhanced light detectors 36 located at the focal plane of the camera.The array of enhanced light detectors 36 consists of a plurality ofarranged enhanced light detectors 31, the enhanced light detectorsoperating as described previously in relation to the embodiment shown inFIG. 5. A controller 41 is coupled to and activates a pulse generator 43with a control signal (shown by line 42). The pulse generator 43 iscoupled to and supplies electric current pulses to each of the enhancedlight detectors 31 of the array 36 and also to a switching means 38(such signals shown by lines 39 and 44 respectively). The electriccurrent pulses provided to the enhanced light detector array 36 and theswitching means 38 are preferably synchronized. The switching means 38is operatively interposed between and coupled with the array of enhancedlight detectors 36 and a digital storage device 40 and is activated bycontrol signals from the controller 41 to the pulse generator 43. Theswitching means 38 transfers the output electric current generated byeachof the enhanced light detectors 31 (shown by line 37), such currentbeing generated by the enhanced light detectors 31 in a similar mannerto the electric current generated by the enhanced light detectors 31 asdescribedfor use in a bar code scanner in Example 1, to the digitalstorage device 40 for processing.

The key element in the SSVC is the pulsed laser sampling amplifier ofthe present invention. With a pulse-operated InGaAsP laser amplifier,the sampling time can be as short as 0.02 nanoseconds with a samplingperiod (or time interval between current pulses) of 0.2 nanoseconds. TheInGaAsP laser amplifiers will be arranged as the input elements in anintegrated optico-electronics array 36, and placed at the focal plane ofthe camera. The "shutter action" is not a mechanical shutter, but isprovided by applying a forward current pulse (shown by line 44) to thelaser amplifierarray 36. The shutter time is much shorter than the pulseduration. The "shutter" opens at the time when the injected electrondensity increases above a critical density for lasing.

The video information is stored in arrays of fast memory 40. While thephoton and electron recovering times impose an absolute limit of no morethan about 5×10⁹ pictures per second, two practical limitationsmayexist. They are:

(i) the time required for moving and/or processing data; and

(ii) heat dissipation.

The camera is highly sensitive. Each laser amplifier has a gain ofapproximately 1000 or above, the gain being limited only by the noisedue to spontaneous emission.

EXAMPLE 3 Application of a Pulse Operated Laser to an Optical FiberCommunications System

FIG. 6 is an illustrative example of an optical communication systemformedin accordance with the present invention, with a single source ofinput light pulses being supplied to a plurality of pulse operated lasersampling light amplifiers 19. The pulse operated laser amplifiers 19 arecontrolled by signals sent from a controller 20. In one embodiment ofthe invention, the control signals from controller 20 are provided tothe current pulse generator means 9 of each sampling or pulsed lightamplifier19 to control the timing of the electric current pulsesgenerated by the pulse generator means. Alternatively, one or morecurrent pulse generators9 may be included in controller 20 to provideelectric current pulses to a plurality of BH laser amplifier structures,such as that shown in FIG. 1.

Three possible operations of this type of system are illustrated by FIG.7.In the first system, illustrated by FIG. 7(a), the pulse operatedlaser amplifiers 19 are being used as signal switches. The controller 20activates only the amplifier associated with output of laser amplifierA. All incoming signals are permitted to pass through amplifier A andnone will be permitted to pass through amplifiers B, C, or D.

FIG. 7(b) depicts a signal distributor in which the laser amplifiers 19arepulsed in sequence. For example, the controller 20 allows the 1st,5th, 9th, . . . pulses to pass through amplifier A; the 2nd, 6th, 10, .. . pulses to pass through amplifier B, and so on. Each amplifier 19receives an equal number of signals equally spaced in time.

FIG. 7(c) depicts a signal distributor of varying densities. More signalpulses are permitted to pass through amplifier A than the otheramplifiers, creating a larger signal density to the output of amplifierA.

While there have been described what are presently believed to be thepreferred embodiments of the invention, those skilled in the art willrealize that changes and modifications may be made thereto withoutdeparting from the spirit of the invention, and it is intended to claimall such changes and modifications as fall within the true scope of theinvention.

What is claimed is:
 1. A pulsed laser sampling light amplifier, whichcomprises:a semiconductor laser light amplifier having an input end andan output end and means defining a resonant cavity situated between andcoupled to the input end and the output end; and means for generatingelectric current pulses, the current pulse generating means beingcoupled to the laser light amplifier to apply electric current pulses tothe light amplifier to inject minority carriers into the resonant cavitythereof; the electric current pulses generated by the pulse generatingmeans being of sufficient magnitude and duration to raise the number ofthe minority carriers above a critical level temporarily after theapplication of each of the current pulses, the current pulses beingsufficiently separated in time for the minority carriers to decay belowthe critical level after the termination of each of the current pulses,said critical level of minority carriers being defined as the minimumnumber of minority carriers above which an intensity of light in theresonant cavity would rise exponentially with time even without furtherlight input.
 2. A pulsed laser sampling light amplifier as defined byclaim 1, wherein the input end and the output end of the light amplifierhave a reflectivity which exceeds 0.1.
 3. A pulsed laser sampling lightamplifier as defined by claim 1, wherein each of the current pulsesgenerated by the pulse generating means includes two peaks which areseparated in time.
 4. A pulsed laser sampling light amplifier as definedby claim 2, wherein each of the current pulses generated by the pulsegenerating means includes two peaks which are separated in time.
 5. Apulsed laser sampling light amplifier as defined by claim 1, wherein thelight amplifier includes a funnel shaped section situated at the inputend thereof and coupled to the resonant cavity.
 6. A pulsed lasersampling light amplifier as defined by claim 1, which further includesmeans for generating a control signal, the control signal generatingmeans being coupled to the current pulse generating means and providinga control signal thereto for controlling the timing of the electriccurrent pulses generated by the pulse generating means.
 7. An enhancedlight detector for converting light into electrical current, whichcomprises:a pulsed laser sampling light amplifier as defined by claim 5;and a diode light detector situated adjacent to the output end of thepulse laser sampling light amplifier, the diode light detector receivinga light output emitted by the laser sampling light amplifier andconverting the light output into electric current.
 8. A method ofoperating a semiconductor laser sampling light amplifier having aresonant cavity, which comprises the step of:applying electric currentpulses to the light amplifier to inject minority carriers into theresonant cavity thereof, the electric current pulses being of sufficientmagnitude and duration to raise the number of the minority carriersabove a critical level temporarily after the application of each of thecurrent pulses, the current pulses being sufficiently separated in timefor the minority carriers to decay below the critical level after thetermination of each of the current pulses, said critical level ofminority carriers being defined as the minimum number of minoritycarriers above which an intensity of light in the resonant cavity wouldrise exponentially with time even without further light input.